Impedance Adjustment Device and Impedance Adjustment Method

ABSTRACT

A high frequency power supply alternately outputs a first AC voltage and a second AC voltage to a plasma generator. The amplitudes of the first AC voltage and the second AC voltage are different from each other. An impedance adjustment device is disposed in midway of the transmission line of the first AC voltage and the second AC voltage. When the AC voltage output from the high frequency power supply is switched to a first AC voltage, a microcomputer changes the capacitance of a variable capacitor circuit to a first target value. When the AC voltage output from the high frequency power supply is switched to a second AC voltage, the microcomputer changes the capacitance of the variable capacitor circuit to a second target value.

CROSS-REFERENCE TO RELATED APPLICATION

This Nonprovisional application claims priority under 35 U.S.C. § 119(a)on Patent Application No. 2019-238643 filed in Japan on Dec. 27, 2019,the entire contents of which are hereby incorporated by reference.

FIELD

The present disclosure relates to an impedance adjustment device and animpedance adjustment method.

BACKGROUND

Japanese Patent Laid-Open Publication No. 2015-90759 discloses a powersupply system in which an AC voltage having a high frequency is outputfrom an AC power supply to a load. In the power supply system describedin Japanese Patent Laid-Open Publication No. 2015-90759, a first ACvoltage and a second AC voltage are alternately output to the load,amplitudes of the first AC voltage and the second AC voltage beingdifferent from each other. An impedance adjustment device is disposed inmidway of the transmission line of the first AC voltage and the secondAC voltage output from the AC power supply to the load. The impedanceadjustment device adjusts the impedance on the load side when viewedfrom the AC power supply.

The impedance adjustment device adjusts the impedance on the load sidewhen viewed from the AC power supply by adjusting the reactance of areactance element disposed in midway of the transmission line. Theimpedance adjustment device calculates the impedance on the load sidebased on the information of both the first AC voltage and the second ACvoltage.

In the impedance adjustment device, it is considered that the impedanceon the load side when viewed from the AC power supply is the calculatedimpedance on the load side. The impedance adjustment device calculates,for example, the reactance of the reactance element at which thereflection coefficient of the AC voltage when viewed from the AC powersupply is zero. The impedance adjustment device determines the targetvalue of the reactance based on the calculated reactance. The impedanceadjustment device changes the reactance of the reactance element to thedetermined target value. Thus, the actual reflection coefficient ismaintained at a small value. So-called impedance matching is performed.As a result, electric power can be efficiently supplied to the loadside.

SUMMARY

The impedance adjustment device described in Japanese Patent Laid-OpenPublication No. 2015-90759 calculates the impedance on the load sidebased on the information of both the first AC voltage and the second ACvoltage. In the impedance adjustment device, it is considered that theactual impedance on the load side is the calculated impedance on theload side. Then, the impedance adjustment device adjusts the reactanceof the reactance element. However, the calculated impedance on the loadside is not the impedance on the load side when the first AC voltage isoutput. In addition, the calculated impedance on the load side is notthe impedance on the load side when the second AC voltage is output.Therefore, in the power supply system described in Japanese PatentLaid-Open Publication No. 2015-90759, the reflection coefficient doesnot become zero. In the power supply system, there is always a reflectedwave that returns to the AC power supply. Therefore, it is desired toefficiently supply electric power to the load side by reducing themagnitude of reflected waves.

The present disclosure has been made in view of such circumstances, andit is an object of the present disclosure to provide an impedanceadjustment device and an impedance adjustment method capable of reducingthe magnitude of reflected waves generated in a power supply system inwhich a first AC voltage and a second AC voltage are alternately outputto a load, amplitudes of the first AC voltage and the second AC voltagebeing different from each other.

An impedance adjustment device according to an aspect of the presentdisclosure is to be disposed in midway of a transmission line of an ACvoltage output from an AC power supply to a load and adjusts animpedance on the load side when viewed from the AC power supply. The ACpower supply alternately outputs a first AC voltage and a second ACvoltage. Amplitudes of the first AC voltage and the second AC voltageare different from each other. The impedance adjustment device includes:a variable impedance circuit; a first changing unit that changes animpedance of the variable impedance circuit to a first target value whenthe AC voltage output from the AC power supply is switched to the firstAC voltage; and the second changing unit that changes the impedance ofthe variable impedance circuit to a second target value when the ACvoltage output from the AC power supply is switched to the second ACvoltage. The second target value is different from the first targetvalue.

An impedance adjustment device according to an aspect of the presentdisclosure further includes: a first acquiring unit that repeatedlyacquires first information regarding the first AC voltage; a firstnumerical value calculation unit that calculates, based on the firstinformation acquired by the first acquiring unit, a first impedance onthe load side when viewed from the AC power supply or a first reflectioncoefficient of the first AC voltage when viewed from the AC powersupply; a first average value calculation unit that calculates anaverage value of a plurality of first impedances or of a plurality offirst reflection coefficients, the plurality of first impedances or theplurality of first coefficients being calculated by the first numericalvalue calculation unit based on a plurality of pieces of firstinformation acquired by the first acquiring unit within a calculationperiod including a plurality of first periods, during the first periodthe first AC voltage being output; and a first determining unit thatdetermines the first target value based on the average value calculatedby the first average value calculation unit.

An impedance adjustment device according to an aspect of the presentdisclosure further includes: a second acquiring unit that repeatedlyacquires second information regarding the second AC voltage; a secondnumerical value calculation unit that calculates, based on the secondinformation acquired by the second acquiring unit, a second impedance onthe load side when viewed from the AC power supply or a secondreflection coefficient of the second AC voltage when viewed from the ACpower supply; a second average value calculation unit that calculates anaverage value of a plurality of second impedances or of a plurality ofsecond reflection coefficients, the plurality of second impedances orthe plurality of second reflection coefficients being calculated by thesecond numerical value calculation unit based on a plurality of piecesof second information acquired by the second acquiring unit within thecalculation period; and a second determining unit that determines thesecond target value based on the average value calculated by the secondaverage value calculation unit. The calculation period includes theplurality of first periods and a plurality of second periods, during thesecond period the second AC voltage being output.

In an impedance adjustment device according to an aspect of the presentdisclosure, the variable impedance circuit includes a plurality ofseries circuits. In each series circuit, a capacitor and a switch areconnected in series. The plurality of series circuits are connected inparallel. The first changing unit and the second changing unit changethe impedance of the variable impedance circuit to the first targetvalue and to the second target value respectively by switching one ormore switches included in the variable impedance circuit on or offseparately.

In an impedance adjustment method according to an aspect of the presentdisclosure, an impedance on the load side when viewed from an AC powersupply is adjusted by changing an impedance of a variable impedancecircuit in a power supply system in which the AC power supplyalternately outputs a first AC voltage and a second AC voltage,amplitudes of the first AC voltage and the second AC voltage beingdifferent from each other. The impedance adjustment method causes acomputer to execute processings of changing the impedance of thevariable impedance circuit to a first target value when an AC voltageoutput from the AC power supply is switched to the first AC voltage; andchanging the impedance of the variable impedance circuit to a secondtarget value when the AC voltage output from the AC power supply isswitched to the second AC voltage, the second target value beingdifferent from the first target.

In the impedance adjustment device and the impedance adjustment methodaccording to the aspect described above, the impedance of the variableimpedance circuit is the first target value while the first AC voltageis output. The impedance of the variable impedance circuit is the secondtarget value while the second AC voltage is output. The first targetvalue is adjusted to, for example, an impedance at which the reflectioncoefficient when viewed from the AC power supply becomes a minimum valuein a case where the first AC voltage is output. The second target valueis adjusted to, for example, an impedance at which the reflectioncoefficient when viewed from the AC power supply becomes a minimum valuein a case where the second AC voltage is output. In this case, since theimpedance on the load side is always adjusted to an appropriateimpedance, the magnitude of reflected waves can be reduced.

In the impedance adjustment device according to the aspect describedabove, the average value of the first impedances is calculated when thecalculation period including a plurality of first periods passes. It isnot necessary to calculate the average value each time the first periodpasses. Therefore, as a circuit for calculating the average value, aninexpensive circuit having a slow calculation speed can be used. Withinthe calculation period, a period for repeatedly acquiring the firstinformation is long. Therefore, when the load is, for example, a plasmagenerator, the first impedance does not change greatly due to minutechanges in plasma in the plasma generator.

In the impedance adjustment device according to the aspect describedabove, the average value of the second impedances is calculated when thecalculation period including a plurality of second periods passes. It isnot necessary to calculate the average value of the second impedanceseach time the second period passes. Therefore, as a circuit forcalculating the average value, an inexpensive circuit having a slowcalculation speed can be used. Within the calculation period, a periodfor repeatedly acquiring the second information is long. Therefore, whenthe load is, for example, a plasma generator, the second impedance doesnot change greatly due to minute changes in plasma in the plasmagenerator.

In the impedance adjustment device according to the aspect describedabove, the variable impedance circuit includes a plurality of seriescircuits. In each series circuit, a capacitor and a switch are connectedin series. These series circuits are connected in parallel. Theimpedance of the variable impedance circuit can be easily changed byswitching the plurality of switches on or off separately.

According to the present disclosure, it is possible to reduce themagnitude of reflected waves generated in the power supply system inwhich the first AC voltage and the second AC voltage are alternatelyoutput to the load, amplitudes of the first AC voltage and the second ACvoltage being different from each other.

The above and further objects and features will move fully be apparentfrom the following detailed description with accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the main configuration of a powersupply system according to Embodiment 1.

FIG. 2 is a timing chart for describing the operation of a highfrequency power supply.

FIG. 3 is a flowchart showing the procedure of a calculation process ofa calculation circuit.

FIG. 4 is a flowchart showing the procedure of the calculation processof the calculation circuit.

FIG. 5 is a block diagram showing the main configuration of amicrocomputer.

FIG. 6 is a flowchart showing the procedure of a capacitance changeprocess.

FIG. 7 is a flowchart showing the procedure of a target valuedetermination process.

FIG. 8 is a timing chart for describing the operation of an impedanceadjustment device.

FIG. 9 is a block diagram showing the main configuration of a powersupply system according to Embodiment 2.

DETAILED DESCRIPTION OF NON-LIMITING EXAMPLE EMBODIMENTS

Hereinafter, the present disclosure will be described in detail withreference to the diagrams showing embodiments thereof.

Embodiment 1 <Configuration of Power Supply System>

FIG. 1 is a block diagram showing the main configuration of a powersupply system 1 according to Embodiment 1. The power supply system 1includes a high frequency power supply 10, a plasma generator 11, a highfrequency detector 12, an impedance adjustment device 13, and asynchronization signal output device 14. The high frequency power supply10 is connected to the plasma generator 11 through a transmission lineTa. The high frequency detector 12 and the impedance adjustment device13 are disposed in midway of the transmission line Ta. The highfrequency detector 12 is located between the high frequency power supply10 and the impedance adjustment device 13. The high frequency powersupply 10 and the plasma generator 11 are grounded.

It is noted that the transmission line Ta indicates a transmission linefrom the high frequency power supply 10 to the plasma generator 11.Therefore, in FIG. 1, the high frequency detector 12 and an inductor 20,which will be described later, are disposed on the transmission line Ta.

The synchronization signal output device 14 outputs a synchronizationsignal configured by a high level voltage and a low level voltage, tothe high frequency power supply 10 and to the impedance adjustmentdevice 13. The high frequency power supply 10 is an AC power supply thatoutputs an AC voltage having a high frequency based on thesynchronization signal input from the synchronization signal outputdevice 14.

FIG. 2 is a timing chart for describing the operation of the highfrequency power supply 10. FIG. 2 shows the transition of the voltage ofthe synchronization signal, the waveform of the AC voltage output fromthe high frequency power supply 10, and the transition of the AC poweroutput from the high frequency power supply 10. For these transitionsand the waveform, the horizontal axis indicates time. In FIG. 2, thehigh level voltage and the low level voltage are indicated by “H” and“L”, respectively. In diagrams other than FIG. 2, the high level voltageand the low level voltage are indicated by “H” and “L”, respectively.

As shown in FIG. 2, the voltage of the synchronization signal isperiodically switched to the low level voltage and to the high levelvoltage. The duty of the synchronization signal is fixed to variousvalues. The duty of the synchronization signal is a ratio occupied by aperiod, during which the synchronization signal indicates a high levelvoltage, in one cycle. In the example of FIG. 2, the duty is 60%.

When the synchronization signal indicates a high level voltage, the highfrequency power supply 10 outputs a first AC voltage having a firstamplitude B1 as its amplitude. When the synchronization signal indicatesa low level voltage, the high frequency power supply 10 outputs a secondAC voltage having a second amplitude B2 as its amplitude. The highfrequency power supply 10 alternately outputs the first AC voltage andthe second AC voltage. The first amplitude B1 and the second amplitudeB2 are fixed values and are set in advance. The first amplitude B1 isdifferent from the second amplitude B2. In the example of FIG. 2, thefirst amplitude B1 is larger than the second amplitude B2. Thefrequencies of the first AC voltage and the second AC voltage are commonfrequencies. The frequencies of the first AC voltage and the second ACvoltage are frequencies belonging to the industrial radio frequency (RF)band. Frequencies belonging to the industrial RF band include 400 kHz, 2MHz, 13.56 MHz, 27.12 MHz, 40.68 MHz, 60 MHz, and the like.

The high frequency power supply 10 outputs the first AC voltage and thesecond AC voltage to the plasma generator 11 through the high frequencydetector 12 and the impedance adjustment device 13. At this time, thefirst AC voltage and the second AC voltage output from the highfrequency power supply 10 are transmitted through the transmission lineTa. The output impedance of the high frequency power supply 10 isexpressed by for example, only the real part. In this case, the outputimpedance is, for example, 50Ω.

The high frequency power supply 10 outputs first AC power P1 byoutputting the first AC voltage. The high frequency power supply 10outputs second AC power P2 by outputting the second AC voltage. Thefirst AC power P1 and the second AC power P2 are fixed values and exceed0 W. The first AC power P1 is different from the second AC power P2. Inthe example of FIG. 2, the first AC power P1 is larger than the secondAC power P2.

The plasma generator 11 shown in FIG. 1 generates plasma by using thefirst AC voltage and the second AC voltage input from the high frequencypower supply 10. When the type of the plasma generator 11 is acapacitive coupling type, the plasma generator 11 includes aplate-shaped upper electrode and a plate-shaped lower electrode. Theplate surface of the upper electrode faces the plate surface of thelower electrode. The first AC voltage and the second AC voltage outputfrom the high frequency power supply 10 are alternately applied to theupper electrode. The lower electrode is grounded. By applying the firstAC voltage and the second AC voltage, plasma is generated between theupper electrode and the lower electrode. The first AC voltage and thesecond AC voltage may be alternately output to the lower electrode. Inthis case, the upper electrode is grounded.

When the type of the plasma generator 11 is an inductive coupling type,the plasma generator 11 includes an inductor. One end of the inductor isgrounded. The first AC voltage and the second AC voltage output from thehigh frequency power supply 10 are alternately applied to the other endof the inductor. Thus, plasma is generated in the inductor.

The plasma generated by the plasma generator 11 is used for processing,such as etching or chemical vapor deposition (CVD). In the plasmagenerator 11, the state of the plasma changes over time while theprocessing is performed. When the state of plasma changes, the impedanceof the plasma generator 11 changes.

In a state in which the high frequency power supply 10 outputs the firstAC voltage, the impedance on the plasma generator 11 side when viewedfrom the high frequency power supply 10 is referred to as a firstimpedance. In the similar state, the reflection coefficient of the firstAC voltage on the plasma generator 11 side when viewed from the highfrequency power supply 10 is referred to as a first reflectioncoefficient. In a state in which the high frequency power supply 10outputs the second AC voltage, the impedance on the plasma generator 11side when viewed from the high frequency power supply 10 is referred toas a second impedance. In the similar case, the reflection coefficientof the second AC voltage on the plasma generator 11 side when viewedfrom the high frequency power supply 10 is referred to as a secondreflection coefficient. The reflection coefficient is a complex number.The absolute value of the reflection coefficient is 0 or more and 1 orless.

Each of the first impedance and the second impedance is one of followingtwo impedances. One impedance is an impedance when the plasma generator11 side is viewed from the output end of the high frequency power supply10. The other impedance is an impedance when the plasma generator 11side is viewed from the input end of the first AC voltage and the secondAC voltage in the impedance adjustment device 13. The input end of theimpedance adjustment device 13 corresponds to the output end of the highfrequency power supply 10. Each of the first impedance and the secondimpedance is a combined impedance of the impedance of the impedanceadjustment device 13 and the impedance of the plasma generator 11.

The high frequency detector 12 periodically detects first parametersregarding the first AC voltage during a first period in which the highfrequency power supply 10 outputs the first AC voltage. The highfrequency detector 12 generates first parameter information indicatingthe detected first parameters. The impedance adjustment device 13acquires the first parameter information from the high frequencydetector 12. Similarly the high frequency detector 12 periodicallydetects second parameters regarding the second AC voltage during asecond period in which the high frequency power supply 10 outputs thesecond AC voltage. The high frequency detector 12 generates secondparameter information indicating the detected second parameters. Theimpedance adjustment device 13 acquires the second parameter informationfrom the high frequency detector 12.

The impedance adjustment device 13 calculates the first impedance or thefirst reflection coefficient based on the first parameter information.The impedance adjustment device 13 calculates the second impedance orthe second reflection coefficient based on the second parameterinformation. The first parameter information and the second parameterinformation correspond to first information and second information,respectively.

As a first example of the first parameters, a first AC voltage, a firstAC current corresponding to the first AC voltage, and a phase differencebetween the first AC voltage and the first AC current can be mentioned.As a second example of the first parameters, forward wave power (orforward wave voltage) and reflected wave power (or reflected wavevoltage) can be mentioned. Here, the forward wave voltage is the firstAC voltage transmitting toward the plasma generator 11. The forward wavepower is the power of the forward wave voltage. The reflected wavevoltage is an AC voltage that is reflected by the plasma generator 11and that transmits toward the high frequency power supply 10. Thereflected wave power is the power of the reflected wave voltage. Thesecond parameters are similar to the first parameters. By replacing thefirst AC voltage with the second AC voltage in the description of theexample of the first parameters, the example of the second parameterscan be described.

The impedance adjustment device 13 adjusts the first impedance and thesecond impedance by changing the impedance of the impedance adjustmentdevice 13. As described above, the impedance adjustment device 13calculates the first impedance or the first reflection coefficient basedon the first parameter information. The impedance adjustment device 13calculates the second impedance or the second reflection coefficientbased on the second parameter information.

Based on the calculation result, the impedance adjustment device 13changes the impedance of the impedance adjustment device 13 so that thefirst impedance becomes a complex conjugate of the output impedance ofthe high frequency power supply 10 during the first period or so thatthe reflection coefficient becomes a minimum value during the firstperiod. When the first impedance cannot be adjusted to the complexconjugate of the output impedance, the impedance adjustment device 13adjusts the impedance of the impedance adjustment device 13 so that thefirst impedance becomes a value closest to the complex conjugate of theoutput impedance.

Based on the calculation result, the impedance adjustment device 13performs, during the second period, the similar change as the impedancechange performed during the first period. By replacing the firstimpedance with the second impedance in the description of the impedancechange of the impedance adjustment device 13 performed during the firstperiod, the impedance change performed during the second period can bedescribed.

<Configuration of Impedance Adjustment Device 13>

The impedance adjustment device 13 includes the inductor 20, a variablecapacitor circuit 21, a capacitor 22, a microcomputer 23, and acalculation circuit 24. The inductor 20 is disposed in midway of thetransmission line Ta. One end of the variable capacitor circuit 21 isconnected to one end of the inductor 20 on the high frequency detector12 side. One end of the capacitor 22 is connected to one end of theinductor 20 on the plasma generator 11 side. The other ends of thevariable capacitor circuit 21 and the capacitor 22 are grounded.

The circuit including the inductor 20, the variable capacitor circuit21, and the capacitor 22 is a π-type circuit. The circuit included inthe impedance adjustment device 13 is not limited to the π-type circuit.The circuit included in the impedance adjustment device 13 may be anL-type circuit, a T-type circuit, or the like. The following circuit canbe mentioned as a first example of the L-type circuit. In this circuit,one end of the variable capacitor circuit 21 is connected to one end orthe other end of a series circuit including the inductor 20 and thecapacitor 22. The other end of the variable capacitor circuit 21 isgrounded. The following circuit can be mentioned as a second example ofthe L-type circuit. In this circuit, one end of the capacitor 22 isconnected to one end or the other end of a series circuit including theinductor 20 and the variable capacitor circuit 21. The other end of thecapacitor 22 is grounded.

The following circuit can be mentioned as an example of the T-typecircuit. In this circuit, the inductor 20 is connected in series to aninductor (not shown). One end of the variable capacitor circuit 21 isconnected to a connection node between the inductor 20 and the inductor.The other end of the variable capacitor circuit 21 is grounded.

Hereinafter, an example in which the impedance adjustment device 13includes a π-type circuit will be described.

The variable capacitor circuit 21 includes n capacitor circuits A1, A2,. . . , An connected in parallel. Here, n is an integer of 2 or more.Each of the capacitor circuits A1, A2, . . . , An includes a capacitor30, a PIN diode 31, and a driving unit 32. In each of the capacitorcircuits A1, A2, . . . , An, one end of the capacitor 30 is connected toone end of the inductor 20. The other end of the capacitor 30 isconnected to the anode of the PIN diode 31. The cathode of the PIN diode31 is grounded. In this manner, the capacitor 30 is connected in seriesto the PIN diode 31. The driving unit 32 is connected to a connectionnode between the capacitor 30 and the PIN diode 31.

The parallelism of the n capacitor circuits A1, A2, . . . , An does notmean strict parallelism, but means substantial parallelism. Therefore,for example, a series circuit of the capacitor circuit A2 and a resistor(not shown) may be connected between both ends of the capacitor circuitA1.

The driving unit 32 applies, to the anode of the PIN diode 31, apositive voltage having reference potential corresponding to a groundpotential. Thus, a forward voltage is applied to the PIN diode 31. Inaddition, the driving unit 32 applies, to the anode of the PIN diode 31,a negative voltage having reference potential corresponding to theground potential. Thus, a reverse voltage is applied to the PIN diode31.

In the PIN diode 31, P-type, I-type, and N-type semiconductor layers arebonded to each other. The I-type semiconductor is an intrinsicsemiconductor. The I-type semiconductor layer is disposed between theP-type semiconductor layer and the N-type semiconductor layer. An anodeand a cathode are provided on the P-type semiconductor layer and theN-type semiconductor layer, respectively. The PIN diode 31 functions asa switch.

When the driving unit 32 applies a forward voltage to the PIN diode 31,the resistance value between both ends of the PIN diode 31 drops to asufficiently small value. As a result, the PIN diode 31 is switched on.When the driving unit 32 applies a reverse voltage to the PIN diode 31,the resistance value between both ends of the PIN diode 31 rises to asufficiently large value. As a result, the PIN diode 31 is switched off.As described above, the driving unit 32 switches the PIN diode 31connected the driving unit 32 on or off. When the PIN diode 31 is ON,the AC voltage can pass through the PIN diode 31. When the PIN diode 31is OFF, the AC voltage cannot pass through the PIN diode 31.

The microcomputer 23 outputs a high level voltage or a low level voltageto the n driving units 32 included in the variable capacitor circuit 21.When the voltage input from the microcomputer 23 is switched from thelow level voltage to the high level voltage, each driving unit 32switches the PIN diode 31 on. When the voltage input from themicrocomputer 23 is switched from the high level voltage to the lowlevel voltage, each driving unit 32 switches the PIN diode 31 off.

When the number of PIN diodes 31 in the ON state is 2 or more, thecapacitance of the variable capacitor circuit 21 is expressed by the sumof the capacitances of the plurality of capacitors 30 connected to theplurality of PIN diodes 31 in the ON state. When the number of PINdiodes 31 in the ON state is 1, the capacitance of the variablecapacitor circuit 21 is expressed by the capacitance of the capacitor 30connected to the PIN diode 31 in the ON state.

The capacitance of the capacitor 30 included in the capacitor circuit Ai(i=1, 2, . . . , n) is expressed by the product of a positive real valueand (i−1)-th power of 2. Therefore, the capacitance of the variablecapacitor circuit 21 can be adjusted at an interval of theabove-described real value. When the real value is 1 pF, the capacitanceof the variable capacitor circuit 21 can be adjusted at an interval of 1pF.

As described above, the variable capacitor circuit 21 is a circuitcapable of changing the capacitance, that is, the impedance. Thevariable capacitor circuit 21 functions as a variable impedance circuit.The capacitance of the variable capacitor circuit 21 corresponds to theimpedance of the variable impedance circuit.

The synchronization signal output device 14 outputs the synchronizationsignal not only to the high frequency power supply 10 but also to themicrocomputer 23 and the calculation circuit 24. The calculation circuit24 acquires the first parameter information and the second parameterinformation from the high frequency detector 12. The first parametersindicated by the first parameter information acquired by the calculationcircuit 24 substantially matches the first parameters at the time ofacquisition. Similarly, the second parameters indicated by the secondparameter information acquired by the calculation circuit 24substantially matches the second parameters at the time of acquisition.The microcomputer 23 outputs, to the calculation circuit 24, a startsignal, which gives an instruction to start the calculation of the firstimpedance and the second impedance.

The calculation circuit 24 is formed by, for example, afield-programmable gate array (FPGA). The calculation circuit 24performs a calculation process for calculating a first average value anda second average value. The first average value is the average value ofthe first impedances or of the first reflection coefficients. The secondaverage value is the average value of the second impedances or of thesecond reflection coefficients. In the calculation process, thecalculation circuit 24 repeatedly acquires the first parameterinformation and the second parameter information until the calculationperiod passes after the start signal is input from the microcomputer 23.The calculation period is k cycles of the synchronization signal. Here,k is an integer of 2 or more. The calculation period includes aplurality of first periods and a plurality of second periods. Thecalculation circuit 24 calculates the first impedance or the firstreflection coefficient each time the first parameter information isacquired. In addition, the calculation circuit 24 calculates the secondimpedance or the second reflection coefficient each time the secondparameter information is acquired. When the calculation period haspassed, the calculation circuit 24 calculates the first average valueand the second average value. The calculation circuit 24 outputs averageinformation indicating the calculated first average value and thecalculated second average value to the microcomputer 23.

When the average information is input from the calculation circuit 24,the microcomputer 23 determines a first target value and a second targetvalue regarding the capacitance of the variable capacitor circuit 21based on the first average value and the second average value indicatedby the average information input from the calculation circuit 24.

As described above, the microcomputer 23 switches the output voltages,which are output to the n driving units 32 included in the variablecapacitor circuit 21, to a high level voltage or to a low level voltage.Thus, the n PIN diodes 31 included in the variable capacitor circuit 21are switched on or off separately. The microcomputer 23 can easilychange the capacitance of the variable capacitor circuit 21 by switchingthe n PIN diodes 31 on or off separately. When the voltage of thesynchronization signal is switched to the high level voltage, themicrocomputer 23 changes the capacitance of the variable capacitorcircuit 21 to the first target value. When the voltage of thesynchronization signal is switched to the high level voltage, the firstperiod starts. When the voltage of the synchronization signal isswitched to the low level voltage, the microcomputer 23 changes thecapacitance of the variable capacitor circuit 21 to the second targetvalue. When the voltage of the synchronization signal is switched to thelow level voltage, the second period starts.

Hereinafter, the operations of the calculation circuit 24 and themicrocomputer 23 will be described in detail.

<Calculation Process of Calculation Circuit 24>

FIGS. 3 and 4 are flowcharts showing the procedure of the calculationprocess of the calculation circuit 24. Here, the calculation process forcalculating the first average value of the first impedances and thesecond average value of the second impedances will be described.

In the calculation process, the calculation circuit 24 determineswhether or not a start signal has been input from the microcomputer 23(step S1). When it is determined that the start signal has not beeninput (S1: NO), the calculation circuit 24 executes step S1 again andwaits until the start signal is input. When it is determined that thestart signal has been input (S1: YES), the calculation circuit 24determines whether or not the voltage of the synchronization signal hasbeen switched from the low level voltage to the high level voltage (stepS2).

When it is determined that the voltage of the synchronization signal hasnot been switched to the high level voltage (S2: NO), the calculationcircuit 24 executes step S2 again, and waits until the voltage of thesynchronization signal is switched from the low level voltage to thehigh level voltage. When it is determined that the voltage of thesynchronization signal has been switched to the high level voltage (S2:YES), the calculation circuit 24 determines whether or not the waitingtime has passed after the voltage of the synchronization signal isswitched to the high level voltage (step S3). The waiting time is afixed value, and is set in advance. The calculation circuit 24 executesstep S3 using, for example, a timer (not shown). In this case, the timermeasures the time that has passed after the voltage of thesynchronization signal is switched to the high level voltage.

When it is determined that the waiting time has not passed (S3: NO), thecalculation circuit 24 executes step S3 again and waits until thewaiting time passes. As described above, when the voltage of thesynchronization signal is switched to the high level voltage, themicrocomputer 23 changes the capacitance of the variable capacitorcircuit 21 to the first target value. By waiting until the waiting timepasses, the calculation circuit 24 waits until the capacitance becomesstable after the capacitance is changed to the first target value. Thewaiting time is sufficiently shorter than the first and second periods.

When it is determined that the waiting time has passed (S3: YES), thecalculation circuit 24 acquires the first parameter information from thehigh frequency detector 12 (step S4), and calculates the first impedancebased on the first parameters indicated by the acquired first parameterinformation (step S5). Then, the calculation circuit 24 determineswhether or not the voltage of the synchronization signal has beenswitched from the high level voltage to the low level voltage (step S6).When it is determined that the voltage of the synchronization signal hasnot been switched to the low level voltage (S6: NO), the calculationcircuit 24 executes step S4 again. The calculation circuit 24 repeatsthe acquisition of the first parameter information and the calculationof the first impedance until the voltage of the synchronization signalis switched from the high level voltage to the low level voltage.

When it is determined that the voltage of the synchronization signal hasbeen switched to the low level voltage (S6: YES), the calculationcircuit 24 determines whether or not the waiting time has passed (stepS7) as in step S3. When a timer is used, the timer measures the timethat has passed after the voltage of the synchronization signal isswitched to the low level voltage.

When it is determined that the waiting time has not passed (S7: NO), thecalculation circuit 24 executes step S7 again and waits until thewaiting time passes. As described above, when the voltage of thesynchronization signal is switched to the low level voltage, themicrocomputer 23 changes the capacitance of the variable capacitorcircuit 21 to the second target value. By waiting until the waiting timepasses, the calculation circuit 24 waits until the capacitance becomesstable after the capacitance is changed to the second target value.

When it is determined that the waiting time has passed (ST-YES), thecalculation circuit 24 acquires the second parameter information fromthe high frequency detector 12 (step S8). Then, the calculation circuit24 calculates the second impedance based on the second parametersindicated by the second parameter information acquired in step S8 (stepS9). After executing step S9, the calculation circuit 24 determineswhether or not the voltage of the synchronization signal has beenswitched from the low level voltage to the high level voltage (stepS10). When it is determined that the voltage of the synchronizationsignal has not been switched to the high level voltage (S10: NO), thecalculation circuit 24 executes step S8 again. The calculation circuit24 repeats the acquisition of the second parameter information and thecalculation of the second impedance until the voltage of thesynchronization signal is switched from the low level voltage to thehigh level voltage.

When it is determined that the voltage of the synchronization signal hasbeen switched to the high level voltage (S10: YES), the calculationcircuit 24 determines whether or not the calculation period has passedafter the calculation circuit 24 determines in step S2 that the voltageof the synchronization signal has been switched to the high levelvoltage (step S11). As described above, the calculation period is kcycles. Therefore, the calculation period passes at the timing when thevoltage of the synchronization signal is switched to the high levelvoltage. The calculation circuit 24 executes step S11 using, forexample, a timer. In this case, the timer measures the time that haspassed after the voltage of the synchronization signal is switched tothe high level voltage in step S2.

When it is determined that the calculation period has not passed (S11:NO), the calculation circuit 24 executes step S4 again to calculate thefirst impedance and the second impedance. When it is determined that thecalculation period has passed (S11: YES), the calculation circuit 24calculates the first average value of the plurality of first impedancescalculated during the calculation period (step S12). After executingstep S12, the calculation circuit 24 calculates the second average valueof the plurality of second impedances calculated during the calculationperiod (step S13). Then, the calculation circuit 24 outputs, to themicrocomputer 23, average information indicating the first average valueand the second average value calculated in steps S12 and S13 (step S14).

After executing step S14, the calculation circuit 24 ends thecalculation process. After the end of the calculation process, thecalculation circuit 24 restarts the calculation process and waits untilthe start signal is input from the microcomputer 23.

As described above, the calculation circuit 24 repeatedly acquires thefirst parameter information and the second parameter information duringthe calculation period. The calculation circuit 24 calculates the firstimpedance based on the acquired first parameter information. Thecalculation circuit 24 calculates the second impedance based on theacquired second parameter information. The calculation circuit 24calculates the average value of the plurality of first impedancescalculated based on the plurality of pieces of first parameterinformation acquired within the calculation period. The calculationcircuit 24 calculates the average value of the plurality of secondimpedances calculated based on the plurality of pieces of secondparameter information acquired within the calculation period. Thecalculation circuit 24 functions as a first acquiring unit, a firstnumerical value calculation unit, a first average value calculationunit, a second acquiring unit, a second numerical value calculationunit, and a second average value calculation unit.

A calculation process for calculating the first average value of thefirst reflection coefficients and the second average value of the secondreflection coefficients is similar to the calculation process forcalculating the first average value of the first impedances and thesecond average value of the second impedances. In the description of thecalculation process for calculating the first impedance and the secondimpedance, the first impedance and the second impedance are replacedwith the first reflection coefficient and the second reflectioncoefficient, respectively. Therefore, the calculation process forcalculating the first reflection coefficient and the second reflectioncoefficient can be described.

The calculation circuit 24 may be configured to include a processor thatexecutes processing. The processor is, for example, a central processingunit (CPU). In this case, in the calculation circuit 24, a computerprogram is stored in a storage unit (not shown), and the processorexecutes the calculation process by executing the computer program.

The computer program may be stored in a storage medium so as to bereadable by the processor of the calculation circuit 24. In this case,the computer program read from the storage medium by a reader (notshown) is written in the storage unit of the calculation circuit 24. Thestorage medium is an optical disk, a flexible disk, a magnetic disk, amagneto-optical disk, a semiconductor memory, or the like. The opticaldisk is a compact disc (CD)-read only memory (ROM), a digital versatiledisc (DVD)-ROM, a Blu-ray (registered trademark) disc (BD), or the like.The magnetic disk is, for example, a hard disk. In addition, a computerprogram may be downloaded from an external device (not shown) connectedto a communication network (not shown), and the downloaded computerprogram may be written in a storage unit.

<Configuration of Microcomputer 23>

FIG. 5 is a block diagram showing the main configuration of themicrocomputer 23. The microcomputer 23 includes input units 40 and 41,output units 42 and 43, a storage unit 44, and a control unit 45. Theseare connected to an internal bus 46. The input unit 40 is also connectedto the synchronization signal output device 14. Each of the input unit41 and the output unit 42 is also connected to the calculation circuit24. The output unit 43 is also connected to the n driving units 32 ofthe n capacitor circuits A1, A2, . . . , An separately.

The synchronization signal output device 14 outputs a synchronizationsignal to the input unit 40. The output unit 42 outputs a start signalto the calculation circuit 24 according to an instruction from thecontrol unit 45. The calculation circuit 24 outputs the averageinformation to the input unit 41. The output unit 43 outputs a highlevel voltage or a low level voltage to the n driving units 32. Theoutput unit 43 separately switches the voltages output to the n drivingunits 32 to a high level voltage or a low level voltage according to aninstruction from the control unit 45.

First target value information indicating the first target value andsecond target value information indicating the second target value arestored in the storage unit 44. The first target value and the secondtarget value indicated by the first target value information and thesecond target value information are updated by the control unit 45. Acomputer program P is stored in the storage unit 44. The control unit 45includes a processor that executes processing. The processor is, forexample, a CPU. The processor of the control unit 45 performs acapacitance change process and a target value determination process inparallel by executing the computer program P. In the capacitance changeprocess, the capacitance of the variable capacitor circuit 21 ischanged. In the target value determination process, the first targetvalue and the second target value are determined.

The computer program P may be stored in a storage medium E so as to bereadable by the processor of the control unit 45. In this case, thecomputer program P read from the storage medium E by a reader (notshown) is written in the storage unit 44 of the microcomputer 23. Thestorage medium E is an optical disk, a flexible disk, a magnetic disk, amagneto-optical disk, a semiconductor memory, or the like. In addition,the computer program P may be downloaded from an external device (notshown) connected to a communication network (not shown). In this case,the downloaded computer program P is written in the storage unit 44.

The number of processors included in the control unit 45 may be 2 ormore. In this case, a plurality of processors may cooperatively performthe capacitance change process and the target value determinationprocess.

<Capacitance Change Process>

FIG. 6 is a flowchart showing the procedure of the capacitance changeprocess. In the capacitance change process, the control unit 45determines whether or not the voltage of the synchronization signaloutput from the synchronization signal output device 14 to the inputunit 40 has been switched from the low level voltage to the high levelvoltage (step S2 l). When it is determined that the voltage of thesynchronization signal has not been switched to the high level voltage(S21: NO), the control unit 45 executes step S21 again, and waits untilthe voltage of the synchronization signal is switched from the low levelvoltage to the high level voltage.

When it is determined that the voltage of the synchronization signal hasbeen switched to the high level voltage (S21: YES), the control unit 45reads the first target value indicated by the first target valueinformation (step S22). Then, the control unit 45 changes thecapacitance of the variable capacitor circuit 21 to the first targetvalue read in step S22 by switching one or more PIN diodes 31 on or offseparately (step S23). Here, the one or more PIN diodes 31 are all thePIN diodes 31 that need to be switched in order to change thecapacitance of the variable capacitor circuit 21 to the first targetvalue. The control unit 45 instructs the output unit 43 to separatelyswitch the output voltages, which are to be output to one or moredriving units 32 corresponding to the one or more PIN diodes 31, to highlevel voltages or to low level voltages. Thus, the control unit 45realizes the switching of one or more PIN diodes 31 to ON or OFF. Whenstep S23 is executed, the first impedance is changed.

After executing step S23, the control unit 45 determines whether or notthe voltage of the synchronization signal output from thesynchronization signal output device 14 to the input unit 40 has beenswitched from the high level voltage to the low level voltage (stepS24). When it is determined that the voltage of the synchronizationsignal has not been switched to the low level voltage (S24: NO), thecontrol unit 45 executes step S24 again, and waits until the voltage ofthe synchronization signal is switched to the low level voltage.

When it is determined that the voltage of the synchronization signal hasbeen switched to the low level voltage (S24: YES), the control unit 45reads the second target value indicated by the second target valueinformation (step S25). Then, in the similar manner as in step S23, thecontrol unit 45 changes the capacitance of the variable capacitorcircuit 21 to the second target value read in step S25 by switching oneor more PIN diodes 31 on or off (step S26). Here, the one or more PINdiodes 31 are all the PIN diodes 31 that need to be switched in order tochange the capacitance of the variable capacitor circuit 21 to thesecond target value. When step S25 is executed, the second impedance ischanged.

After executing step S26, the control unit 45 ends the capacitancechange process. After the end of the capacitance change process, thecontrol unit 45 performs the capacitance change process again and waitsuntil the voltage of the synchronization signal is switched from the lowlevel voltage to the high level voltage.

As described above, in the capacitance change process, the control unit45 changes the capacitance of the variable capacitor circuit 21 to thefirst target value when the voltage of the synchronization signal isswitched to the high level voltage. When the voltage of thesynchronization signal is switched to the high level voltage, the ACvoltage output from the high frequency power supply 10 is switched tothe first AC voltage. When the voltage of the synchronization signal isswitched to the low level voltage, the control unit 45 changes thecapacitance of the variable capacitor circuit 21 to the second targetvalue. When the voltage of the synchronization signal is switched to thelow level voltage, the AC voltage output from the high frequency powersupply 10 is switched to the second AC voltage. The control unit 45functions as a first change unit and a second change unit.

<Target Value Determination Process>

FIG. 7 is a flowchart showing the procedure of the target valuedetermination process. The control unit 45 performs the target valuedetermination process when the average information is input from thecalculation circuit 24 to the input unit 41. Here, the target valuedetermination process performed when the average information indicatingthe first average value of the first impedances and the second averagevalue of the second impedances is input will be described.

In the target value determination process, the control unit 45calculates a first capacitance, at which the first impedance becomes acomplex conjugate of the output impedance of the high frequency powersupply 10, based on the first average value of the first impedancesindicated by the average information input to the input unit 41 (step S3l). Then, the control unit 45 determines a first target value based onthe first capacitance calculated in step S31 (step S32). Here, the firsttarget value is a capacitance that can be realized in the variablecapacitor circuit 21. The first target value is a capacitance thatmatches the calculated first capacitance or is closest to the calculatedfirst capacitance. The control unit 45 also functions as a firstdetermining unit.

Then, the control unit 45 updates the first target value indicated bythe first target value information to the first target value determinedin step S32 (step S33). After executing step S33, when the first periodstarts, the capacitance of the variable capacitor circuit 21 is changedto the first target value determined in step S32.

Then, the control unit 45 calculates a second capacitance, at which thesecond impedance becomes a complex conjugate of the output impedance ofthe high frequency power supply 10, based on the second average value ofthe second impedances indicated by the average information input to theinput unit 41 (step S34). Then, the control unit 45 determines a secondtarget value based on the second capacitance calculated in step S34(step S35). Here, the second target value is a capacitance that can berealized in the variable capacitor circuit 21. The second target valueis a capacitance that matches the calculated second capacitance or isclosest to the calculated second capacitance. The control unit 45 alsofunctions as a second determining unit.

Then, the control unit 45 updates the second target value indicated bythe second target value information to the second target valuedetermined in step S35 (step S36). After executing step S36, when thesecond period starts, the capacitance of the variable capacitor circuit21 is changed to the second target value determined in step S35.

After executing step S36, the control unit 45 instructs the output unit42 to output a start signal to the calculation circuit 24 (step S37).Thus, the calculation circuit 24 repeatedly calculates the firstimpedance and the second impedance in the calculation process. Thecalculation circuit 24 calculates a first average value of the pluralityof calculated first impedances and a second average value of theplurality of calculated second impedances.

After executing step S37, the control unit 45 ends the target valuedetermination process.

The target value determination process performed when the averageinformation indicating the first average value of the first reflectioncoefficients and the second average value of the second reflectioncoefficients is input is similar to the target value determinationprocess performed when the average information indicating the firstaverage value of the first impedances and the second average value ofthe second impedances is input. In step S31 of the target valuedetermination process of the first reflection coefficient and the secondreflection coefficient, the first capacitance at which the firstreflection coefficient becomes zero is calculated. In step S35 of thetarget value determination process of the first reflection coefficientand the second reflection coefficient, the second capacitance at whichthe second reflection coefficient becomes zero is calculated. When theoutput unit 42 outputs a start signal to the calculation circuit 24, thecalculation circuit 24 repeatedly calculates the first reflectioncoefficient and the second reflection coefficient. The calculationcircuit 24 calculates a first average value of the plurality ofcalculated first reflection coefficients and a second average value ofthe plurality of calculated second reflection coefficients.

<Operation of Impedance Adjustment Device 13>

FIG. 8 is a timing chart for describing the operation of the impedanceadjustment device 13. FIG. 8 shows the transition of the voltage of thesynchronization signal. FIG. 8 further shows the processes performed bythe calculation circuit 24, the microcomputer 23, and the driving unit32 in chronological order. Time is shown on the horizontal axes. In FIG.8, the calculation of the first impedance or the first reflectioncoefficient is referred to as a first calculation. The calculation ofthe second impedance or the second reflection coefficient is referred toas a second calculation. FIG. 8 shows an example in which k is 2, thatis, an example in which the calculation period is two cycles of thesynchronization signal.

As shown in FIG. 8, when the voltage of the synchronization signal isswitched from the low level voltage to the high level voltage, themicrocomputer 23 instructs one or more driving units 32 to switch one ormore PIN diodes 31 on or off. Thus, the capacitance of the variablecapacitor circuit 21 is changed to the first target value indicated bythe first target value information. When the voltage of thesynchronization signal is switched from the high level voltage to thelow level voltage, the microcomputer 23 instructs one or more drivingunits 32 to switch one or more PIN diodes 31 on or off. Thus, thecapacitance of the variable capacitor circuit 21 is changed to thesecond target value indicated by the second target value information.

Therefore, during the first period in which the high frequency powersupply 10 outputs the first AC voltage, the capacitance of the variablecapacitor circuit 21 is the first target value indicated by the firsttarget value information. During the second period in which the highfrequency power supply 10 outputs the second AC voltage, the capacitanceof the variable capacitor circuit 21 is the second target valueindicated by the second target value information.

As shown in FIG. 8, when the voltage of the synchronization signal isswitched from the low level voltage to the high level voltage after theoutput unit 42 of the microcomputer 23 outputs a start signal to thecalculation circuit 24, the calculation period starts. The calculationcircuit 24 repeatedly calculates the first impedance or the firstreflection coefficient during the calculation period. In addition, thecalculation circuit 24 repeatedly calculates the second impedance or thesecond reflection coefficient during the calculation period. Themicrocomputer 23 repeatedly calculates the first impedance or the firstreflection coefficient after the waiting time passes from the switchingof the voltage of the synchronization signal to the high level voltagein each of the k first periods. The microcomputer 23 repeatedlycalculates the second impedance or the second reflection coefficientafter the waiting time passes from the switching of the voltage of thesynchronization signal to the low level voltage in each of the k secondperiods.

The time for switching one or more PIN diodes 31 on or off is referredto as a switching time. The time required for the capacitance of thevariable capacitor circuit 21 to stabilize after switching one or morePIN diodes 31 on or off is referred to as a stabilization time. Thewaiting time is longer than the total time of the switching time and thestabilization time. Therefore, the first parameters indicated by thefirst parameter information acquired by the calculation circuit 24 arefirst parameters detected by the high frequency detector 12 in a statein which the capacitance of the variable capacitor circuit 21 is stable.Similarly the second parameters indicated by the second parameterinformation acquired by the calculation circuit 24 are second parametersdetected by the high frequency detector 12 in a state in which thecapacitance of the variable capacitor circuit 21 is stable.

When the calculation period passes, the calculation circuit 24calculates a first average value based on the plurality of firstimpedances or the plurality of first reflection coefficients calculatedduring the calculation period. In addition, the calculation circuit 24calculates a second average value based on the plurality of secondimpedances or the plurality of second reflection coefficients calculatedduring the calculation period. The calculation circuit 24 outputs, tothe input unit 41 of the microcomputer 23, average informationindicating the calculated first average value and the calculated secondaverage value. The calculation circuit 24 stops the calculation afteroutputting the average information. The calculation circuit 24 restartsthe calculation when the voltage of the synchronization signal isswitched from the low level voltage to the high level voltage after thestart signal is input again.

When the average information is input to the input unit 41, the controlunit 45 of the microcomputer 23 determines a first target value and asecond target value based on the first average value and the secondaverage value indicated by the input average information. The controlunit 45 updates the first target value and the second target valueindicated by the first target value information and the second targetvalue information to the determined first target value and thedetermined second target value. After the update is performed, the firsttarget value or the second target value is changed.

<Effect of Impedance Adjustment Device 13>

As described above, the capacitance of the variable capacitor circuit 21is adjusted to the first target value during the first period in whichthe high frequency power supply 10 outputs the first AC voltage. Duringthe second period in which the high frequency power supply 10 outputsthe second AC voltage, the capacitance of the variable capacitor circuit21 is adjusted to the second target value. In this case, the impedanceon the plasma generator 11 side when viewed from the high frequencypower supply 10 is always adjusted to an appropriate impedance.Therefore, since the magnitude of reflected waves is reduced, electricpower can be efficiently supplied to the plasma generator 11.

In addition, the calculation circuit 24 calculates an average value ofthe first impedances and an average value of the second impedances whenthe calculation period including the plurality of first periods and theplurality of second periods passes. It is not necessary to calculate theaverage value of the first impedances each time the first period passes.It is not necessary to calculate the average value of the secondimpedances each time the second period passes. Therefore, as thecalculation circuit 24, an inexpensive circuit having a slow calculationspeed can be used.

In addition, within the calculation period, a period during which thecalculation circuit 24 repeatedly acquires the first parameterinformation is long. Therefore, the first impedance does not changegreatly due to minute changes in the state of plasma in the plasmagenerator 11. Similarly, within the calculation period, a period duringwhich the calculation circuit 24 repeatedly acquires the secondparameter information is long. Therefore, the second impedance does notchange greatly due to minute changes in the state of plasma in theplasma generator 11.

<Notes>

There is no problem as long as the variable capacitor circuit 21functions as a circuit that can change its own impedance. Therefore, oneof following two circuits may be used instead of the variable capacitorcircuit 21. One circuit is a circuit that can change its own resistancevalue and reactance. The other circuit is a circuit that can change itsown reactance. The number of first impedances calculated by thecalculation circuit 24 during the first period may be one. The number ofsecond impedances calculated by the calculation circuit 24 during thesecond period may also be one.

The number of cycles of the synchronization signal included in thecalculation period may be one. The number of cycles of thesynchronization signal included in the calculation period is the valueof k. When the number of cycles of the synchronization signal is 1, thenumber of first impedances calculated by the calculation circuit 24during the first period is 2 or more. In addition, the number of secondimpedances calculated by the calculation circuit 24 during the secondperiod is also 2 or more. In the impedance adjustment device 13, theprocess performed by the calculation circuit 24 may be performed by thecontrol unit 45 of the microcomputer 23. In this case, parameterinformation is output from the high frequency detector 12 to themicrocomputer 23 of the impedance adjustment device 13.

Also in a period other than the calculation period, the calculationcircuit 24 may repeat the calculation of the first impedance and thesecond impedance or the calculation of the first reflection coefficientand the second reflection coefficient. In the configuration in which thecalculation of the first impedance and the second impedance is repeated,the first impedance and the second impedance calculated in the periodother than the calculation period are not used. In the configuration inwhich the calculation of the first reflection coefficient and the secondreflection coefficient is repeated, the first reflection coefficient andthe second reflection coefficient calculated in the period other thanthe calculation period are not used. There is no problem as long as thePIN diode 31 functions as a switch. Therefore, a field effect transistor(FET), a bipolar transistor, a relay contact, and the like may be usedinstead of the PIN diode 31. The load to which the high frequency powersupply 10 outputs an AC voltage is not limited to the plasma generator11, and may be, for example, a non-contact power transmission device.

Embodiment 2

In Embodiment 1, when the type of the plasma generator 11 is acapacitive coupling type, the first AC voltage and the second AC voltageare alternately applied to one of the upper electrode and the lowerelectrode in a state in which the other of the upper electrode and thelower electrode is grounded. However, an AC voltage may be applied toboth the upper electrode and the lower electrode.

Hereinafter, the differences between Embodiments 1 and 2 will bedescribed. Other configurations excluding the configuration describedbelow are common to those in Embodiment 1. Therefore, the componentscommon to those in Embodiment 1 are denoted by the same referencenumerals. The description of the common components will be omitted.

<Configuration of Power Supply System 1>

FIG. 9 is a block diagram showing the main configuration of a powersupply system 1 according to Embodiment 2. The power supply system 1according to Embodiment 2 includes a high frequency power supply 10, aplasma generator 11, a high frequency detector 12, an impedanceadjustment device 13, and a synchronization signal output device 14, asin Embodiment 1. Their connections are similar to those in Embodiment 1.

The power supply system 1 according to Embodiment 2 further includes ahigh frequency power supply 50, a high frequency detector 51, and animpedance adjustment device 52. The plasma generator 11 according toEmbodiment 2 includes an injection container 60, an upper electrode 61,and a lower electrode 62. FIG. 9 shows a cross section of the injectioncontainer 60. The injection container 60 is formed in a box shape andhas two through holes. Gas is injected into the injection container 60through one of the through holes. Substance is discharged from the otherthrough hole. Each of the upper electrode 61 and the lower electrode 62is formed in a plate shape. The upper electrode 61 and the lowerelectrode 62 are housed in the injection container 60. The plate surfaceof the upper electrode 61 faces the plate surface of the lower electrode62.

The upper electrode 61 is connected to the impedance adjustment device13. In the impedance adjustment device 13, as shown in FIG. 1, when aπ-type circuit is formed by the inductor 20, the variable capacitorcircuit 21, and the capacitor 22, one end of the inductor 20 on thecapacitor 22 side is connected to the upper electrode 61. The highfrequency power supply 10, the plasma generator 11, the high frequencydetector 12, the impedance adjustment device 13, and the synchronizationsignal output device 14 operate in the similar manner as inEmbodiment 1. Therefore, the first AC voltage and the second AC voltageare alternately applied to the upper electrode 61.

The high frequency power supply 50 is connected to the lower electrode62 of the plasma generator 11 through a transmission line Tb. The highfrequency detector 51 and the impedance adjustment device 52 aredisposed in midway of the transmission line Tb. The high frequencydetector 51 is located between the high frequency power supply 50 andthe impedance adjustment device 52. The high frequency power supply 50is grounded.

The high frequency power supply 50 is an AC power supply that outputs anAC voltage having a high frequency. The frequency of the AC voltageoutput from the high frequency power supply 50 is a frequency belongingto the industrial RF band. Frequencies belonging to the industrial RFband are 400 kHz, 2 MHz, 13.56 MHz, 27.12 MHz, 40.68 MHz, 60 MHz, andthe like. The high frequency power supply 50 outputs the AC voltage tothe lower electrode 62 of the plasma generator 11 through the highfrequency detector 51 and the impedance adjustment device 52. At thistime, the AC voltage output from the high frequency power supply 50 istransmitted through the transmission line Tb. The output impedance ofthe high frequency power supply 50 is expressed by, for example, onlythe real part. In this case, the output impedance is, for example, 50Ω.

The amplitude of the AC voltage output from the high frequency powersupply 50 is fixed. The frequency of the AC voltage may match thefrequencies of the first AC voltage and the second AC voltage. Thefrequency of the AC voltage output from the high frequency power supply50 may be different from the frequencies of the first AC voltage and thesecond AC voltage. Regarding the plasma generator 11, it is assumed thatthe first AC voltage and the second AC voltage are alternately appliedto the upper electrode 61 and the AC voltage is applied to the lowerelectrode 62 while the gas is injected into the injection container 60.In this case, plasma is generated between the upper electrode 61 and thelower electrode 62. An object W to be processed is disposed on the platesurface of the lower electrode 62 on the upper electrode 61 side. In theplasma generator 11, processing such as etching or CVD is performed onthe object W using plasma.

In Embodiment 2, the synchronization signal output device 14 outputs asynchronization signal not only to the high frequency power supply 10,the high frequency detector 12, and the impedance adjustment device 13but also to the high frequency detector 51 and the impedance adjustmentdevice 52. When the AC voltage output from the high frequency powersupply 10 shown on the upper side of FIG. 9 is changed, the impedance onthe plasma generator 11 side when viewed from the high frequency powersupply 50 changes in the similar manner as the impedance on the plasmagenerator 11 side when viewed from the high frequency power supply 10.

The impedance on the plasma generator 11 side when viewed from the highfrequency power supply 50 is one of following two impedances. Oneimpedance is an impedance when the plasma generator 11 side is viewedfrom the output end of the high frequency power supply 50. The otherimpedance is an impedance when the plasma generator 11 side is viewedfrom the input end of the AC voltage in the impedance adjustment device52. The input end of the impedance adjustment device 52 corresponds tothe output end of the high frequency power supply 50. The impedance onthe plasma generator 11 side when viewed from the high frequency powersupply 50 is a combined impedance of the impedance of the impedanceadjustment device 52 and the impedance of the plasma generator 11.

The high frequency detector 51 and the impedance adjustment device 52operate in the similar manner as the high frequency detector 12 and theimpedance adjustment device 13, respectively. Regarding the highfrequency detector 51 and the impedance adjustment device 52, the firstparameters are parameters regarding the AC voltage output from the highfrequency power supply 50 during the first period in which the highfrequency power supply 10 outputs the first AC voltage. The secondparameters are parameters regarding the AC voltage output from the highfrequency power supply 50 during the second period in which the highfrequency power supply 10 outputs the second AC voltage.

Here, as a first examples of the parameters, an AC voltage and an ACcurrent transmitted through the high frequency detector 51 and a phasedifference between the AC voltage and the AC current can be mentioned.As a second example of the parameters, forward wave power (or forwardwave voltage) and a reflected wave power (or reflected wave voltage) canbe mentioned. Here, the forward wave voltage is an AC voltagetransmitting from the high frequency power supply 50 to the plasmagenerator 11. The forward wave power is the power of the forward wavevoltage. The reflected wave voltage is an AC voltage that is reflectedby the plasma generator 11 and that transmits toward the high frequencypower supply 50. The reflected wave power is the power of the reflectedwave voltage.

<Effect of Impedance Adjustment Device 13>

Also in Embodiment 2, the impedance adjustment device 13 achieves thesimilar effects as in Embodiment 1.

<Notes>

In Embodiment 2, two electrodes to which the high frequency powersupplies 10 and 50 apply AC voltages may be the lower electrode 62 andthe upper electrode 61, respectively. In addition, the power supplysystem 1 according to Embodiment 2 may include the high frequency powersupply 10, the high frequency detector 12, the impedance adjustmentdevice 13, and the synchronization signal output device 14 instead ofthe high frequency power supply 50, the high frequency detector 51, andthe impedance adjustment device 52. In this case, one high frequencypower supply 10 alternately applies the first AC voltage and the secondAC voltage to the upper electrode 61. The other high frequency powersupply 10 alternately applies the first AC voltage and the second ACvoltage to the lower electrode 62.

In this case, the rising points or the falling points of twosynchronization signals output from the two synchronization signaloutput devices 14 match each other. The rising point is a point in timeat which the voltage indicated by the synchronization signal is switchedfrom the low level voltage to the high level voltage. The falling pointis a point in time at which the voltage indicated by the synchronizationsignal is switched from the high level voltage to the low level voltage.In addition, for the two synchronization signals, one period is m timesthe other period. Here, m is a natural number. Even in the power supplysystem 1 configured as described above, the impedance adjustment device13 achieves the similar effects as in Embodiment 1.

It is noted that, as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural referents unless thecontext clearly dictates otherwise.

The technical features (configuration requirements) described inEmbodiments 1 and 2 can be combined with each other. By combining thetechnical features (configuration requirements) described in Embodiments1 and 2 with each other, new technical features can be formed.

It should be considered that Embodiments 1 and 2 disclosed are examplesin all points and not restrictive. The scope of the invention is definedby the claims rather than the meanings set forth above, and is intendedto include all modifications within the scope and meaning equivalent tothe claims.

What is claimed is:
 1. An impedance adjustment device that is to bedisposed in midway of a transmission line of an AC voltage output froman AC power supply to a load and that adjusts an impedance when viewedfrom the AC power supply comprising: a variable impedance circuit; and aprocessor, wherein the AC power supply alternately outputs a first ACvoltage and a second AC voltage, amplitudes of the first AC voltage andthe second AC voltage are different from each other, the processorchanges an impedance of the variable impedance circuit to a first targetvalue when the AC voltage output from the AC power supply is switched tothe first AC voltage, the processor changes the impedance of thevariable impedance circuit to a second target value when the AC voltageoutput from the AC power supply is switched to the second AC voltage,and the second target value is different from the first target value. 2.The impedance adjustment device according to claim 1, furthercomprising: a calculation circuit, wherein the calculation circuitrepeatedly acquires first information regarding the first AC voltage,the calculation circuit calculates, based on the acquired firstinformation, a first impedance when viewed from the AC power supply or afirst reflection coefficient of the first AC voltage when viewed fromthe AC power supply, the calculation circuit calculates an average valueof a plurality of first impedances or of a plurality of first reflectioncoefficients, the plurality of first impedances or the plurality offirst coefficients being calculated based on a plurality of pieces ofacquired first information within a calculation period including aplurality of first periods, during the first period the first AC voltagebeing output, and the processor determines the first target value basedon the average value calculated by the calculation circuit.
 3. Theimpedance adjustment device according to claim 2, wherein thecalculation circuit repeatedly acquires second information regarding thesecond AC voltage, the calculation circuit calculates, based on theacquired second information, a second impedance when viewed from the ACpower supply or a second reflection coefficient of the second AC voltagewhen viewed from the AC power supply, the calculation circuit calculatesan average value of a plurality of second impedances or of a pluralityof second reflection coefficients, the plurality of second impedances orthe plurality of second reflection coefficients being calculated basedon a plurality of pieces of acquired second information within thecalculation period, the processor determines the second target valuebased on the average value calculated by the calculation circuit, andthe calculation period includes the plurality of first periods and aplurality of second periods, during the second period the second ACvoltage being output.
 4. The impedance adjustment device according toclaim 1, wherein the variable impedance circuit includes a plurality ofseries circuits, in each series circuit, a capacitor and a switch areconnected in series, the plurality of series circuits are connected inparallel, and the processor changes the impedance of the variableimpedance circuit to the first target value and to the second targetvalue by switching one or more switches included in the variableimpedance circuit on or off separately.
 5. An impedance adjustmentmethod for adjusting an impedance when viewed from an AC power supply bychanging an impedance of a variable impedance circuit in a power supplysystem in which the AC power supply alternately outputs a first ACvoltage and a second AC voltage, amplitudes of the first AC voltage andthe second AC voltage being different from each other, the impedanceadjustment method causing a computer to execute processings of: changingthe impedance of the variable impedance circuit to a first target valuewhen an AC voltage output from the AC power supply is switched to thefirst AC voltage; and changing the impedance of the variable impedancecircuit to a second target value when the AC voltage output from the ACpower supply is switched to the second AC voltage, the second targetvalue being different from the first target.